The present invention relates generally to a visual defect simulation (VDS) system and, more particularly, to a system and method for simulating potential, patient-specific, visual field defects that may result from a prescribed process.
Scotomata, regions of blindness in the visual field, can be caused by injury to a portion of the visual system. For example, the treatment of tumors and arteriovenous malformations in the occipital cortex and therapeutic temporal lobectomy for relief of epilepsy can result in scotomata as a side-effect. Depending on the extent and location of cortical damage, the resultant scotomata can severely impact various activities of daily living. The effects of a particular scotoma can be difficult to imagine without direct experience, especially since a scotoma is retinopically fixed, that is, it moves with the eyes. Moreover, small or peripheral scotomata can go undetected by the patient. For example, the naturally occurring blind spot, where the optic nerve passes through the retina, is difficult for most people to detect without special instruction. However, when scotomata occur nearer to the center of gaze, the loss of even a few degrees of visual angle can have a very noticeable effect. In these cases, patients may find that the scotomata significantly affects activities such as reading or driving. Such losses of visual function can profoundly affect an individual's personal and professional life. Reading, which depends on high acuity central vision, is critically sensitive to foveal scotomata. Furthermore, more extensive peripheral scotomata may preclude safe driving and other activities such as biking, skiing, or walking in crowded public areas and, in some cases, may even lead to depression.
In clinical practice, it is acknowledged that invasive surgical procedures involving cortical visual pathways can sometimes cause partial vision loss as a side-effect. To date, there are no widely accepted procedures for predicting such side-effects or for simulating the predicted vision defect for a patient who must make critical decisions about the desirability of surgery or surgical extent versus the resulting quality of life due to potential visual defect.
The overall task of creating a system for scotoma prediction and simulation involves several challenges. For example, in order to present a realistic and clinically useful simulation, the unique retinotopic organization of visual cortex in and around a potential site of pathology should be precisely measured for each patient, individually. Next, this information should be combined with a proposed treatment plan in order to predict any regions of induced vision loss. Furthermore, the predicted field loss should be simulated in a realistic manner. Ultimately the accuracy of the prediction should be verifiable by comparing the predicted pattern of vision loss with the actually observed pattern of vision loss in individual patients.
A few scotoma simulators have been developed but are generally unable to meet these requirements. For example, some visual defect simulators have been created that used one or two-dimensional image-stabilizing systems to simulate a vision defect. However, these systems do not account for the unique retinotopic organization of each patient's visual cortex nor do they account for the effects of a particular plan for brain surgery. Therefore, while these systems may present a fairly realistic simulation of a defect in a patient's visual field, the simulation does not allow the patient to experience a potential/predicted visual defect that is specific to the patient and a proposed form of treatment. That is, the simulation does not provide any individualized, patient-specific information about the potential consequences of a proposed medical procedure.
The advent of functional magnetic resonance imaging (fMRI) provides a potential solution to these problems. Conventional fMRI detects changes in cerebral blood volume, flow, and oxygenation that locally occur in association with increased neuronal activity induced by a sensory, motor or cognitive task. As described in U.S. Pat. No. 5,603,322, an MRI system can be used to acquire signals from the brain over a period of time. As the brain performs a task, these signals are modulated synchronously with task performance to reveal which regions of the brain are involved in performing the task. Much research has been done to identify tasks, such as specific visual stimulations, that stimulate brain functions that are readily amenable to fMRI detection.
In this regard, functional magnetic resonance imaging has been used to create cortical maps of the retinotopic organization of human visual cortex. As described by DeYoe et al. in “Functional Magnetic Resonance Imaging (fMRI) Of The Human Brain”, Journal of Neuroscience Methods, 54 (1994) 171-187, an fMRI scan sequence can be performed while providing a visual stimulation to the subject. Specifically, an optical system can be employed to provide a visual stimulus that changes in a prescribed manner during an FMRI scan sequence. Different regions of the retina can be stimulated and the regions in the visual cortex that respond can be measured by fMRI images and then mapped.
For each patient, fMRI can be used to map the unique functional topography (retinotopy) of visual cortex in and near a potential site of surgery. Using this information, it is possible to predict what portions of the visual field are likely to be affected by a particular surgical approach. Even with this estimate, it can still be difficult to verbally describe how this will affect the patient.
It would be desirable to have a system and method to provide a subject with a first-hand, simulated experience of a potential/predicted visual defect that is specific to the subject and the proposed form of treatment.